Hydrogen is an essential material of the hydrogenation process, especially hydrocracking process in refinery, and its purity exerts a tremendous influence on the manufacturing and operating expenses of the hydrogenation unit as well as other reaction performances including the yield of the target products and the operational cycle of catalysts. The process of conventional high-pressure hydrocracking is illustrated in FIG. 5, from which it can be seen that the purity of hydrogen entering into the reactor is dependent on the purity of new hydrogen and the purity of recycle hydrogen. Light hydrocarbon components, methane for example, which are produced during the process of hydrogenation, may be accumulated and thus decrease the purity of recycle hydrogen as they cannot be effectively separated from the recycle hydrogen in the high-pressure separator. A conventional solution for solving the problem is to release part of the recycle hydrogen and at the same time supplement some new hydrogen with high purity so as to keep the concentration of recycle hydrogen at a level such as about 85%. If the concentration of recycle hydrogen in the high-pressure separator can be effectively increased to over 95%, then hydrogen release would be avoided; meanwhile a significant economic benefit can be achieved. For newly established equipment, the designing pressure of its devices can be decreased due to the increased concentration of recycle hydrogen. In the case that the partial pressure of hydrogen needed by the reaction is fixed, the increase of the concentration of hydrogen can decrease the whole reacting pressure and therefore lower the equipment investment of the whole reaction system comprising reactor(s), heating oven(s), heat exchanger(s), cooler(s), high pressure separator(s) and pipes & lines, etc. After the concentration of recycle hydrogen is increased and the total operating pressure of the hydrogenation reactor is decreased, both the lift head of the adapted pump and the compression ratio of the compressor are lowered, and thus the cost of the equipment is cut down. For the equipment that is already under operation, the increase of the concentration of the recycle hydrogen is able to improve the production capacity of the whole reaction system, and it is an important measure of renovation to equipment whose production capacity is to be increased. Additionally, the increase of the hydrogen partial pressure can raise the removal ratio of sulphur and nitrogen, and thus boost the transformation efficiency of raw oil and augment the yield of light oils (gasoline, kerosene, etc.).
Currently, in the state-of-the-art of the pressure swing absorption technology (PSA), its low operation pressure is not suitable for the hydrocracking unit because generally, the pressure of the recycle hydrogen in it is very high. Membrane separation technology, another technology for separation of gas mixtures, is suitable for the separation of high pressure gases, but the pressure of the obtained hydrogen is too low and to solve the problem, before entering the hydrocracking reactor, it should be pressurized hugely and therefore, more energy is consumed, making the technology not so ideal either.